Various applications such as regulating solar heat gain in buildings may use optical methodologies to control the flow of radiant energy (e.g., light and heat). For example, photodarkening materials which have been commonly used in sunglass lenses to selectively attenuate incoming light when stimulated by ultraviolet (“UV”) radiation, may be incorporated into windows. Such materials can be used to regulate the internal temperature of a structure by darkening to attenuate bright sunlight, and by becoming transparent again to allow artificial light or diffuse daylight to pass through unimpeded. Such systems are passive and self-regulating, requiring no external signal other than ambient UV light in order to operate. However, because they are controlled by UV light rather than by temperature, such systems are of limited utility in temperature-regulating applications. For example, they may block wanted sunlight in cold weather as well as unwanted sunlight in hot weather.
In another example of controlling the flow of radiant energy, a system may use thermodarkening materials, which may change color and may increase the amount of light absorbed by the material as the temperature of the material increases or decreases past a predetermined value. For example, Pletotint Corporation of West Olive, Mich. produces a thermodarkening material which can be laminated between two sheets of glass and incorporated into a window.
In addition, electrodarkening filters such as electrically controlled liquid crystal devices have been incorporated into windows. These have the drawback of requiring continuous power to operate, and requiring substantial infrastructure (wiring, switches, sensors, control systems, etc.) as part of their installation. Furthermore, such devices are based on, and use the same basic technology as, LCD video displays. In essence an electrodarkening window filter is a black and white video display with a single gigantic pixel. The liquid crystal in LCD video displays is designed to have a very high “clearing point” (the temperature at which the LC changes phase and becomes an uncontrolled, disorganized, isotropic liquid), to prevent the display from going black under normal operating temperature and light levels. The goal of display design for many years has been to develop liquid crystal formulations that meet other critical design goals such as switching speed with clearing points that are as high as possible to allow the display to run at higher temperatures without this failure mode, and electrodarkening window filters that incorporate commercially available LC mixtures share this trait.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.